Understanding Rust Error E0770

Rust Error E0770: Const parameter references other generic parameters

This error occurs when a const parameter in a Rust function references other generic parameters instead of a concrete type. Consider the following erroneous code example:

fn main() {
  fn foo<T, const N: T>() {} // error!

Here, the const parameter N is of type T, which is a generic parameter. This is not allowed by the Rust compiler.

Resolution: Use a concrete type for the const parameter

To fix this error, replace the generic type T with a concrete type for the const parameter. In most cases, usize is the appropriate type to use. Consider the following corrected code example:

fn main() {
  fn foo<T, const N: usize>() {}

In this example, we've replaced the type of const parameter N with usize, a concrete type. Now the code is valid and compiles without any errors.


When dealing with Rust Error E0770, remember the following key points:

1. A const parameter cannot reference other generic parameters.

2. Instead of using a generic parameter, use a concrete type, such as usize, for the const parameter.

By following these guidelines, you can resolve Rust Error E0770 and write valid Rust code without any issues.